Description of problem:
In order to configure a display, that has been connected using any displaylink(usb) adapter, you can use the dialog in "System Settings/Display and Monitor". Recently mounted displays as not activated. When you activate them, the display gets dark does not get out of it unless you unplug the USB-displaylink adapter from your computer.
Under the cinnamon (i believe also gnome) that works without any problem and you get you monitors running. Some weeks ago this was also possible with KDE. 

Version-Release number of selected component (if applicable):
- system softe fully updated F27
- and display driver from https://github.com/displaylink-rpm/displaylink-rpm

How reproducible:
Any time!

Steps to Reproduce:
1. connect the usb device with you computer 
2. in the dialog "System Settings/Display and Monitor" you will notice some new monitors
3. adjust the monitor-positiion/resolution and activate the monitor(s) and subtmit the changes pressing ok
4. all screens gets dark and will not come back, unless you do not drop the USB-device your computer 
Actual results:


Expected results:
My changes should be accepted, similar to the config in cinnamon and gnome

Additional info:
There is a work-a-round:

1) use cinnamon (gnome) and perform the monitor configuration, 
2) log out and 
3) log-in again with the selected KDE/Plasma session. You can now perform the monitor configuration as you would expect it.